With the growth in the consumer printer market, inkjet printing has become a broadly applicable technology for supplying small quantities of liquid to a surface in an image-wise way. Both drop-on-demand and continuous drop devices have been conceived and built. Whilst the primary development of inkjet printing has been for graphics using aqueous based systems with some applications of solvent based systems, the underlying technology is being applied much more broadly.
There is a general trend of formulation of inkjet inks toward pigment based ink. This generates several issues that require resolution. Further, for industrial printing technologies, i.e. employing printing as a means of manufacture, the liquid formulation may contain hard or soft particulate components that are inherently difficult to handle with inkjet processes.
In a continuous inkjet process a stream of droplets is generated by a droplet generator. Often this droplet generator is an orifice in a thin plate through which liquid, an ink, is forced under pressure to form a liquid jet. It is well known that such a free jet is unstable to perturbations and will disintegrate into a series of droplets through the Rayleigh-Plateau instability. On average this disintegration occurs at a particular wavelength (approximately nine times the radius of the jet). It is also well understood that perturbing the jet via, for example, pressure fluctuations will regularise the jet breakup so that a continuous stream of regularly sized droplets is created. These droplets are conventionally charged via an electrode placed in close proximity to the point of breakup of the jet and subsequently deflected by an electrostatic field. The deflection causes drops to either fall on the substrate to be printed or to be captured and recirculated for re-use. There are many designs of nozzles for such a device. U.S. Pat. No. 4,727,379 describes a resonant cavity energised with a piezo electric device for use as a CIJ droplet generator, U.S. Pat. No. 5,063,393 describes a similar double cavity device and U.S. Pat. No. 5,491,499 describes a simple nozzle with piezo perturbation.
A new continuous inkjet device based on a MEMs formed set of nozzles has been recently developed (see U.S. Pat. No. 6,554,410). In this device a liquid ink jet is formed from a pressurized nozzle. One or more heaters are associated with each nozzle to provide a thermal perturbation to the jet. This perturbation is sufficient to initiate break-up of the jet into regular droplets. By changing the timing of electrical pulses applied to the heater large or small drops can be formed and subsequently separated into printing and non-printing drops via a gaseous cross flow. Although the droplets formed are regular, they nevertheless have a small velocity variation. As the drops travel from the breakoff point their position relative to each other therefore changes. At some distance from the breakoff point this position variation is large enough that neighbouring drops touch and coalesce. In a continuous inkjet device this would then lead to a sorting error or a placement error. Therefore minimisation of velocity variation is imperative.
When a liquid flows across a surface, the velocity of the liquid at or close to the solid surface is zero. In a long pipe the maximum liquid velocity is found in the centre of the pipe and the velocity profile across the pipe is parabolic. This is referred to as Poiseiulle flow. However, on entry to a pipe there is a finite distance, the entry region, where the flow field adopts that consistent with the pipe geometry. In the terminology of fluid mechanics there is a boundary layer that forms and grows until it is the size of the pipe at which point fully developed flow is achieved. The boundary layer thickness may be calculated as
                    δ        =                                            μ              ⁢                                                          ⁢              x                                      ρ              ⁢                                                          ⁢              U                                                          (        1        )            where δ is the boundary layer thickness (m), μ is the liquid viscosity (Pa·s), x is the distance from the start of the pipe (m), ρ is the liquid density (kg/m3) and U the liquid velocity (m/s). The nozzle in an inkjet droplet generator is a very short pipe i.e. too short for fully developed flow to be achieved. Therefore only a boundary layer thickness of liquid next to the nozzle wall is sheared.
Many modern inkjet ink formulations use pigments, a coloured particulate. The advantages of these are well known in the art, in particular providing for better colour gamut and greater lifetime of the printed image. The science of particulates dispersed within liquids, colloid science, is well known. If the particle size is small enough and the density low enough, then Brownian motion is sufficient to cause the particles to remain suspended in the liquid rather than settle out. For inkjet inks, the particulates used usually fulfil this requirement, though there are inventions to allow for inks that do settle e.g. U.S. Pat. No. 6,817,705 B1. More recently metallic particulates have been used which, because of their density, can settle more easily. Particulates may be spherical in shape, but most often are not. Nevertheless, methods to measure the size of particles are often based on measuring the diffusion constant and then from the Stokes-Einstein relation recovering the particle diameter. This process thereby leads to an effective particle diameter that is defined as the equivalent spherical particle that would behave in the same hydrodynamic way and is therefore referred to as the hydrodynamic diameter. Most often the manufacturing process for pigment particulates leads to a distribution of effective particle diameters, referred to as polydispersity. A common way of combining particle diameters to form an average which is relevant for the present invention is to form the volume average thus,
                              d          eff                =                              (                                                            ∑                  j                                ⁢                                                      d                    j                    3                                    ⁢                                      ϕ                    j                                                                              ϕ                total                                      )                                1            /            3                                              (        2        )                                          ϕ          total                =                              ∑            j                    ⁢                      ϕ            j                                              (        3        )            
where deff is the volume average effective particle diameter in nanometers (nm), dj is the particle diameter (nm) of population j and φj is the volume fraction of population j. This can of course be generalised for a continuous distribution of particle diameters,
                              d          eff                =                              (                                                            ∫                  0                  ∞                                ⁢                                                      d                    3                                    ⁢                                      ϕ                    ⁡                                          (                      d                      )                                                        ⁢                                      ⅆ                    d                                                                              ϕ                total                                      )                                1            /            3                                              (        4        )                                          ϕ          total                =                              ∫            0            ∞                    ⁢                                    ϕ              ⁡                              (                d                )                                      ⁢                          ⅆ              d                                                          (        5        )            where φ(d) is the fraction of particles with diameter between d and d+dd.
When a particle is placed in a liquid under shear it will experience a force directed up the shear gradient, i.e. from high shear regions to low shear regions. This is the well known Magnus effect. It will for example cause particulates to be directed toward the centre of a channel or pipe.
There are numerous known methods and devices relating to the formation and use of droplets. For example U.S. Pat. No. 6,713,389 describes placing multiple discrete components on a surface for the purpose of creating electronic devices.